Coatings Technology Handbook Episode 1 Part 6 pptx

19-6 Coatings Technology Handbook, Third Edition19.5 Die Adjustment as It Relates to Manifold Design Precise coating weight control depends on the stability of the total system and on op

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19-6 Coatings Technology Handbook, Third Edition

19.5 Die Adjustment as It Relates to Manifold Design

Precise coating weight control depends on the stability of the total system and on operator experience Die performance will be at its best when a balance of pressure and flow is reached at the lip area This balance is hard to define; changes in materials and rate will affect it We can best describe this through the voice of an operator when he talks about the die being “jumpy or nervous” in one case and “lazy or unresponsive” in the other In the case of the “jumpy” adjusting die, the pressure balance is too high; with the “lazy” one, pressure is too low We will refer to this performance parameter as the operating window When a die must be profiled excessively to improve end flow, the operating window will vary across the die; therefore, the die adjusting characteristics will differ (e.g., jumpy center, lazy ends) This effect is magnified in automatic control When a die is set up at 0.010 in opening in the center and 0.020

in at ends to achieve balanced flow, a 3% change in opening at the ends is 0.0006 in., and in the center

it is a 0.0003 in change As we develop more dynamic computer programs to respond faster or to predetermine a target point, uniform die response becomes very important

19.6 Coat Weight Adjustment

Base coat weight is controlled by pump and line speed Transverse area coat weight is a function of the lip gap adjustment It is important to remember that the pump will always deliver a given amount of fluid to the web When the operator adjusts the lip, material is not taken away — it is only moved from one place to another This difference between roll or knife coating and slot die coating is often overlooked

by the operator

19.7 Adhesive Selection

Adhesives formulated for roll coating, in some cases, are not compatible with slot die heads This is because roll coaters are high shear devices, whereas slot dies develop low shear Any fluid that requires very high shear to create smoothing will not perform well on a low shear coating head The shearing device or lip face can be modified to change shear level in two ways: changing the face-to-web gap (see the shear level, as frequent changes of adhesive and formulation are common

19.8 Die Steel and Piping Selection

Recent developments and reformulations of adhesives have led to highly corrosive or aggressive fluids The die steel or plating must be carefully selected to ensure chemical compatibility, machinability, and reasonable cost We find the majority of problems centering around highly acidic adhesives A pH level

of 4 will severely attack a chrome-plated die and render it unusable in a matter of 2 to 3 months Several stainless steels offer reasonable prices and a highly corrosion-resistant makeup

19.9 Proximity versus Contact Coating

Traditional die designs require being pushed into the backup roll to produce even coating distribution (contact coating) Utilizing modern manifold technology, we can reduce the roll contact pressure signif-icantly and, therefore, reduce roll damage and lip wear Most important, when this application technique

is chosen, the flex lip concept can be used

In any system, it is of utmost importance to have inserts that are quickly replaceable, a lip step differential that can be easily changed, and the capability to increase the shear without having the infeed

DK4036_book.fm Page 6 Monday, April 25, 2005 12:18 PM

Figure 19.6) and changing the face length (Figure 19.7) Provision should be made for ease of changing

lip rotate into and make contact with the web and roll (Figure 19.8)

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Slot Die Coating for Low Viscosity Fluids 19-9

2 The angle of contact depends on lip profile, die-to-roll gap, roll hardness, roll diameter, lip step difference, lip profile, and material spreading tendency It is impossible to predict this angle without all these factors and in-depth experience

3 Reduced contact pressure is necessary to reduce roll and die lip damage

4 Because roll deflection is very hard and expensive to eliminate, a die that can be deflected or bent

to conform to roll variance is required if roll deflection is a problem It is common to use a steel roll backing up the elastomer roll to lessen the deflection problem, and with heated adhesives to help cool the elastomer roll

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for the best and most reproducible coating surface

19.10.2 Lip Profiling

Lip shape and the relative position of the lead and trailing wipers to each other are of utmost importance

in today’s coating technology

In some cases, using today’s high technology coatings, a uniform and proper level of wiping action is required to produce satisfactory coating As a result of the shear thinning characteristics in today’s more difficult adhesives, proper profiling and angle to attack will produce a smooth and even coating Any variance in lip profile will create differential wiping, causing an uneven appearance Because, with modern coating heads, uniform distribution is not a function of the lip face, we can confine the lip face to a single function — namely, creating a proper environment at lay-down

No hard data are available on lip face design as the interrelation between roll diameter, roll hardness, line speed, substrate, lip design, and adhesive viscosity characteristics come into play There seem to be two technical camps One group adheres to the flat, fixed wiping lip, with a differential step between the infeed lip and the wiping, or outfeed, lip The other group tends to favor a rotating or fixed rod in the wiping area This rod style design is as old as die coating, and several patents have been issued

19.10.3 Die Support Design and Operation

The interrelationship of die and mounting require that the two units act as one, both being equally important Absolutely necessary to the successful operation of a slot coating head are the items in the Application and utilization of existing equipment designs will determine the best die-to-roll position Operator convenience and the ability to view the point of laydown are very important It is our opinion The support bed must be completely rigid and vibration-free Any sag or bow in the die will create problems in the die-to-roll alignment For this reason, we do not recommend that the die be supported from its ends On heated dies, the designer must also allow for thermal growth while maintaining die straightness

19.10.4 Support and Adjustment System Design Specifications

(contouring) adjustments for roll warp correction On heated dies, insulation should be provided between die frame, and allowance made for die growth

straightness during operation and adjustment

Item 3 in Figure 19.10: An “X” in/out adjustment utilizing machine tool slideways, with fast air actuation and hydraulic soft stop cushions is used A micro stop correction adjustment with dial indicator

or LVDT should be provided for each end

Item 4 in Figure 19.10: This “Y” adjustment is provided for roll axis position alignment

Angle of attack of the die to roll must be pivoted around the pivot point indicated in Figure 19.8

series of design specifications indicated in Figure 19.10 and explained in Section 19.9.4

that mounting position A in Figure 19.9 is the most advantageous, and position B is the least

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Slot Die Coating for Low Viscosity Fluids 19-11

arrangement, with indicator markings in degrees Note that the angle of attack movement is centered at the die lip (coating contact) point

19.10.5 Die-to-Roll Positioning

Flexibility and repeatability are primary requirements Difficult adhesives, speeds, and substrates will require different setup positions, and the ability to vary the die-to-web position easily with exact repeat-ability is of prime importance The support frame must allow in/out movement and angle of attack adjustment (see Figure 19.10)

In/out adjustment will have two functions: (a) fast movement with 5 to 8 in (130 to 200 mm) of travel, allowing lip cleaning; and (b) micro in/out to fine-tune roll-to-lot gap distance This adjustment should allow differential end-to-end gapping

The in/out adjustment must be on a straight line, always moving directly at the roll centerline (Pivot-style mountings are not recommended.)

19.10.6 Angle of Attack Position Adjustment

To create different shear levels for proper film forming at varying speeds, the angle of attack must be adjustable This movement should be accurate and smooth, and movement should cover about ±5°

It is important that when one positioning point is adjusted, the other roll-to-die relationships remain

19.10.7 Lip Opening Setup

Depending on materials and laydown, a 0.008 to 0.012 in even lip opening gap (Figure 19.11) should

be set before start-up and adjusted for proper flow and laydown after start-up

The lip opening setting adjusts coat weight thickness, not die-to-roll gap Roll coaters will have difficulty with this, as they have traditionally used roll-to-roll gap as the coat weight adjustment

FIGURE 19.11 Lip opening setup.

lip opening

Item 5 in Figure 19.10: This is an angle of attack positioning adjusted through a rack-and-pinion gear

unaltered Angle of attack adjustment must pivot about the point indicated in Figure 19.8

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19.10.8 Die-to-Roll Gap Setup

The distance from die to roll or substrate is, in general, determined by web thickness and the viscosity

of the fluid to be applied The more clearance that can be maintained, the less damage there may be due

to positioning or start-up mistakes

We suggest, as a rule of thumb, that a clearance equal to the substrate thickness be set between substrate and infeed lip face This distance may be less for materials of very low viscosity or for hard roll to lip face

19.11 Backup Roll Design

Processors and equipment manufacturers alike would like to utilize steel backup rolls to improve runout (T.I.R.) and to mitigate heat transfer problems In some cases on lab or narrow production systems, steel rolls have been successful In most cases, however, steel rolls are not as forgiving as an elastomer roll and therefore have not yet been accepted for production We expect this to change as the technology matures

Elastomer rolls have improved over the past several years to allow the precise roll-to-lip conformation absolutely required for proximity coating When specifying an elastomer roll, the following items must

be carefully considered:

• Runout tolerance

• Release characteristics

• Heat transfer

• Hardness

• Hardness uniformity

• Roll deformation (bow)

• Resistance to attack by the coating or cleaning agent

We see the use of rolls covered with urethane or Viton being most evident with diameters of around

300 mm and hardness of up to 90 durometer, Shore A Runout tolerances of 0.0005 in and better are being achieved

The elastomer roll will be deformed by a given width web, and rolls must be provided to match web width changes Spare rolls are also necessary, as damage often occurs

19.12 Automatic Control

19.12.1 Die Control

All commercially acceptable automatic die adjustment systems available today use flexible lip and heated does not seem to be greatly affected The greatest confusion surrounds the effect of pure mechanical response time and how it relates to process analysis and its relation to response All systems to date simply read the variation from target and make a corrective response Polymer lot-to-lot differences, temperature changes, in-plant drafts, and many other factors that affect gauge have made it impossible to anticipate flow changes Anticipation-based programs can be used in product changes, however, if known effects will happen over a relatively fixed period or on start-up

During a production run, if a variance is seen, make sure that it is not a short-term effect, gone in the time it would take to make a change We also may want to determine the variance trend Only after careful analysis of the problem can we make a die adjustment The time from discovery until an adjust-ment takes effect varies from line to line, however start-up to ±5% control, assuming the total system has reached some stability, will be 10 to 20 min, and ±3% control in 15 to 30 min

to smooth materials Make sure the gap is from web to lip face, as shown earlier (Figure 19.6), not from

bolt arrangements (see Figure 19.12) Minor disagreements exist on details, but total overall performance

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Thickness variations are converted to lip opening correction by increasing or decreasing power to the individual lip bolt control blocks to trim variations to a minimum

The key to the transverse thickness control is a microprocessor-based controller that is tied into the conventional computer control system

On ambient operation dies, care must be taken to isolate the heat from the Autoflex bolts from the die body

19.12.2 Die-to-Roll Position Adjustment System

The ability to repeat the original roll-to-die setup position is critical during start-ups and normal web splice coating interruption

During operation, minor changes in die position may have to be made to accommodate roll expansion, changes in adhesive viscosity and smoothability, and substrate thickness variance An automatic posi-tioning device is available that will allow continuous adjustment, if necessary, of the web to the lip face This is accomplished through a device similar to the Autoflex die bolt adjustment system A heating and cooling device is installed in the manual adjustment system (U.S Patent 3,940,221) for die-to-roll gap setup Heating or cooling of this device will expand or contract the steel block and increase or decrease the die-to-roll gap A usable method for monitoring smoothing must be employed and interfaced with the Autoflex computer

19.13 Deckling

Deckling may be necessary to reduce die width or to make stripe coatings Be careful when attempting this, to make sure that shim materials are soft We recommend Teflon/filled, Teflon/aluminum, foil/ aluminum shim stock, or soft brass For room-temperature applications, an adhesive/foam/adhesive mounting tape works very well

does not clamp into the lip and can be changed very quickly

If excessive force is applied when the deckle is clamped into the lip, the lip will distort, causing lip wear or uneven coating at this point (Figure 19.14B)

Deckling or stripe coating cannot be used in a system featuring the rotating rod lip design

19.13.1 Air Entrapment behind Deckling

When the die ends are deckled in, it is common for air to become trapped in this area This air is slowly released, causing voids in the coating at the edges A purge port with shutoff valve should be installed at the die ends to eliminate this problem

19.14 Die Cleanup

In most cases, it is important to change coatings or coating formulation frequently Therefore, it is necessary to be able either to purge out the system or to clean the die completely This holds true for the complete system, including pump filters and piping

Purging is the easiest and most common method; but extreme care is necessary to streamline all flow areas to eliminate dead areas

The opening and cleaning of a die can be an easy 30 min experience or a 3 to 4 day nightmare Slot die designs differ greatly; some are simple two-piece designs, while others have complex assemblies

It is common to have a dual pumping and piping system, allowing quick changeover from one coating

to the next, cleanup then taking place after the line is up and running

In many applications, a rake-type device may be used for stripe coating (Figure 19.14A) This device

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20

Extrusion Coating with Acid Copolymers and

Lonomers

20.1 Product Considerations 20-1 20.2 End-Use Considerations 20-3 20.3 Processing Conditions 20-5

20.1 Product Considerations

Acid copolymers and ionomers are high-performance resins that offer adhesion, heat seal, and barrier acrylic and methacrylic acid copolymers, the two types of acid copolymer commercially available in the United States at this time The presence of the methyl side group in the methacrylic acid copolymers results in some subtle differences between the two resin types, but they can be regarded as equivalent after allowing for the difference in molecular weight of the comonomers For example, a 10 wt% acrylic acid copolymer is equivalent to a 12 wt% methacrylic acid copolymer in carboxyl group content group with either sodium or zinc ions Because the neutralization reaction results in a substantial increase

in melt viscosity, it is commonly referred to as ionic cross-linking

In both acid copolymers and ionomers, the melt and solid state properties are strongly influenced by are almost 10 times stronger than the intramolecular forces in the nonpolar polyethylenes

The ionomers are distinguished by the combination of hydrogen bonding and interchain ionic forces perhaps an order of magnitude stronger than the hydrogen bonds As a consequence, the ionomers have

a wide spectrum of melt and solid state properties, including better hot tack and grease resistance than acid copolymers of equivalent acid content

With the acid copolymers, melt index and acid content are the major variables available to the resin producer Because melt index is a measure of melt viscosity, it is related primarily to the processing characteristics of the resin The resins now used for extrusion coating applications fall in the 5 to 15 melt index range to accommodate a broad field of processing needs

Resins with acid contents of 3 to 15% are currently available on the market The effects of increasing acid content are as follows:

• Better foil adhesion

• Better hot tack

Donald L Brebner

E I du Pont de Nemours &

Company

Foil Adhesion • Heat Seal Characteristics Melt Temperatures • Other Considerations

properties markedly superior to those of conventional polyethylenes Figure 20.1 shows the structure of

Ionomers (Figure 20.2) are derived from acid copolymers by partial neutralization of the carboxyl

intramolecular hydrogen bonding, as illustrated in Figure 20.3 The forces involved in hydrogen bonding

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Hot tack strength is the ability of a heat seal to remain together when a force is applied while it is still

in the molten state This is a critical property in vertical form-fill-seal applications, in which the product

is loaded immediately after the seal is made It is also critical in any high speed packaging operation in which the package is exposed to some form of abuse before the seal has cooled

One method of measuring hot tack strength is the Du Pont spring test A series of springs of different thicknesses and widths at the narrowest point provides varying levels of spring tension (Figure 20.5) pressure of 40 psi The 3 sec dwell time allows an accurate measurement of the interfacial seal temperature

by using a very fine thermocouple hooked up to a rapid response recorder

When the heat seal bars are released, the springs apply an instantaneous force to the seal The separation

of the 1 in wide seal is measured to the nearest tenth of an inch The results are plotted as the force required to obtain 20% seal separation as a function of temperature

content in comparison with LDPE LDPE, a completely nonpolar material, has very low hot tack strength

FIGURE 20.4 Heat seal strength versus bar temperature (1 mil coatings on 30 lb kraft paper).

FIGURE 20.5 Spring for Du Pont hot tack tester.

2000

1500

1000

500

9% Acid Ionomer 9% Acid Copolymers LDPE

Seal Interface Temperature, ° F

8"

1"

3"

Figure 20.6 shows how the spring is located inside the sample The sample is heat sealed for 3 sec at a

Figure 20.7 shows the hot tack characteristics of an ionomer and acid copolymers of equivalent acid

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21

Porous Roll Coater

21.1 Introduction 21-1 21.2 Extrusion Porous Roll System 21-1

21.1 Introduction

Recent progress had been made in silicone-coated products that are curable by electron beam (EB) and products, in which the release levels range between 25 and 50 g per 25 mm wide strip, typical viscosities

of the materials tested are in the range of 500 to 1000 cp at room temperature These silicone products are manufactured by Th Goldschmidt, in West Germany, and Lord Corporation, in Pennsylvania, and other companies The UV products are either one-part premixed/ready-to-use materials, or two-com-ponent products that require nitrogen inerting to overcome surface smear and to achieve a complete cure at web speeds up to 30 m/min per each UV lamp A schematic diagram of the nitrogen inerting lamp) The EB products are also premixed/ready-to-use, and as with the UV chemistries, they also require nitrogen inerting to obtain a full cure at typical web speeds of 200 m/min The energy dosage is

21.2 Extrusion Porous Roll System

21.2.1 Development

One recently developed extrusion roll coating system incorporates a slot nozzle coating head, located transferred to an adjacent “applicating roll,” which in turn, contacts the coating web for fluid transfer The relative speed ratio between the “nozzle roll” and the “applicating roll” is approximately 1:30 Reports on trials have noted that the curable coatings possess poor shear properties and that the speed ratio between the rolls is somewhat dependent on this limitation The shear properties can be improved

by heating the rolls, but this method is not always productive, with the result that the coated web has the appearance of small blotches, 1 to 2 mm or larger, or lateral bands (chatter) of coating, rather than

a smooth, uniform coating Likewise, conventional roll coaters that contain multiple rolls experience a and dilatant characteristics Viscosity of dilatant fluids increases with increasing shear rate, as shown in surface conditions, silicone product properties, and the method of applications

Most coating processes are able to apply these coatings between 1.3 to 1.6 g/m2 within industry standards; however, the type of web material greatly influences the final coating weight Our research to reexamine the current methods for applying curable silicones enabled us to develop an alternative method

Frederic S McIntyre

Acumeter Laboratories, Inc.

Development • Details and Disadvantages

ultraviolet (UV) Advantages of UV and EB converting processes are shown in Table 21.1 Of these new

process is shown in Figure 21.1 The UV lamps used for curing are rated at 300 W/in (120 W/cm per

approximately 2 megarads (Mrad) A schematic diagram of an EB processor is shown in Figure 21.2

adjacent to a slow speed “nozzle roll” (see Figure 21.3) The fluid is coated onto the “nozzle roll” and

similar effect (see Figure 21.4) This phenomenon is associated with fluids that have poor flow properties

Figure 21.5 The actual coat weight applied in either case is influenced by the web substrate material,

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Porous Roll Coater 21-5

FIGURE 21.6 Porous roll coater.

FIGURE 21.7 Diagram of a porous roll.

FIGURE 21.8 Rotary screen printer.

Web

Laminating Roll Applicating Roll

Web Reference Pickup

to Synchronize Pump Speed to Web Speed

3-Way Valve

Digital Pump Drive

Filter

Positive Displacement Metering Pump Supply Feed

Return Feed

Rotary Union

Porous Roll Hopper

Reservoir Cavity

Outer Wall

Reservoir Cavity

Inner Supply Roll

Single Supply Port and Rotary Union

Porous Stainless Steel Sintered Metal Outer Shell

Surface Preparation which Determines Application Pattern

End Cap

Single Width

Rotary Screen Cylinder

Interior Doctor Blade

Printing Fluid Ink

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